Increased yields and catalyst life in catalytic hydrocracking



Aug. 11, 1964 INCREASED YIELDS AND CATALYST LIFE IN CATALYTIC HYDROCRACKING F. e. CIAPETTA ETAL TIME, HRS.

INVENTO R5 Finn/f f1. dime/fa Harry L amrad/ fill/am [f (ram/00d BY W 9. A W

ATTORN United States Patent INCREASED YIELDS AND CATALYST LIFE IN CATALYTIC HYDROCRACKING Frank G. Ciapetta, Silver Spring, Md., and Harry L.

Coonradt, Woodbury, and William E. Garwood, Haddonfield, N .J., assignors to Socony Mobil Oil Company, Inc., a corporation of New York Filed Oct. 16, 1959, Ser. No. 846,844

3 Claims. (Cl. 208112) This application is a continuation-in-part of application Serial Number 661,796, filed May 27, 1957, now abandoned.

This invention relates to the catalytic hydrocracking of petroleum hydrocarbon stocks. It is more particularly concerned with a technique for increasing catalyst life and yields of gasoline and light fuel oil in a hydrocracking process that is carried out in the presence of supported platinium or palladium series metals. In catalytic hydrocracking it is conventional to arrange the catalyst as a fixed bed of solid particles over Which the charge is passed under suitable reaction conditions. Over the course of time a carbonaceous contaminant, generally termed coke, builds up on the catalyst and the catalyst is said to age. A point is eventually reached Where this aging has proceeded to an extent such that the yield and quality of the products has deteriorated so materially that economics dictate that the process be shut down and the coke removed from the catalyst. Coke removal is usually effected by burning. It is, of course, recognized that this regeneration time is nonproductive, since no useful products are obtained during this period. Therefore, any technique which lengthens the permissible on-stream time between regenerations and thereby cuts down the required frequency of regenerations will be of significant economic importance.

In this country the primary and most desired products of any cracking operation are gasoline and light fuel oil. It will therefore also be recognized that any technique which increases the yield of gasoline and light fuel oil from a cracking operation will be of tremendous economic significance.

It has now been found that the on-strearn time of a hydrocracking process between regenerations and the gasoline and light fuel oil yield of such a process can both be increased by a method which is simple and yet commercially feasible. It has been discovered that addition of sulfur compounds to a charge stock during the initial on-stream period of a process for hydrocracking over a specified class of catalysts will increase both the on-stream time between regenerations, and the yield of gasoline and light fuel oil.

Accordingly, it is an object of the present invention to provide an improved catalytic hydrocracking process. Another object is to provide an improved process of catalytic hydrocracking in the presence of hydrogen. A further object is to provide an improved hydrocracking process that is carried out in the presence of hydrogen and of platinum or palladium-containing catalyst- A specific object is to provide a method for increasing catalyst life and gasoline and light fuel oil yields in a hydracracking process that is carried out in the presence of hydrogen and of platinumor palladium-containing catalyst. A particularly specific object of the invention is to provide a method for increasing catalyst life and gasoline and light fuel oil yields in a hydrocracking process that is carried out in the presence of hydrogen on platinumor pal- "ice ladium-containing catalyst that involves regulating the amount of sulfur in the reactor during the initial onstream period of the process.

Other objects and advantages of the present invention will become a parent to those skilled in the art from the following detailed description considered in conjunction with the drawings, of which:

FIGURE 1 presents the graphic relationship between the volume percent conversion into products boiling at temperatures below about 410 F. and the time on-stream when a relatively high platinum content catalyst is used in cracking of a typical charge stock of relatively low sulfur content and of a typical charge stock having a relatively high sulfur content, and

FIGURE 2 presents the graphic relationship between the volume percent conversion into products boiling at temperatures below about 410 F. and the number of hours on-stream when a typical gas oil of relatively low sulfur content is cracked in the presence of a platinumcontaining catalyst of a relatively high platinum content and of a platinum-containing catalyst of a relatively low platinum content.

In general, the present invention provides a process for converting a hydrocarbon fraction having an initial boiling point of at least about 400 F. and a sulfurcontent of less than 4% by Weight, to produce lower boiling products. This fraction is contacted with a catalyst comprising between about 0.05 and about 10 percent, by weight of the catalyst, of at least one noble metal of the platinum and palladium series deposited upon a synthetic composite of refractory oxides of at least two elements of Groups IIA, IIIB and IV of the Periodic Arrangement of Elements having an Activity Index of at least 25 under hydrocracking reaction conditions. During the period of time which extends from the initial contacting of the fraction with the catalyst to a time within the range 0.5 hour to 14 days after said initial contacting, sulfur compounds are added to the fraction in an amount equivalent to a sulfur content in said fraction within the range 0.05 to 5% by weight of said fraction. The addition of sulfur compounds is then discontinued and conversion of the fraction is continued without addition of sulfur compounds until such time as the catalyst becomes sufiiciently aged to require regeneration.

Throughout the specification and claims the term conversion is intended unless otherwise indicated, to be a generic term for the amount of products boiling at temperatures lower than about 410 F. (-recycle), of gasoline, or of Diesel fuel obtained in the process. It is expressed in terms of the volume percent of the initial charge which is transformed in the process. The amount of product boiling at temperatures lower than about 410 F. is obtained by subtracting the volume percent of cycle stock (Diesel fuel) from 100 percent, i.e., from the initial 'vollnne of the charge. The expression, 100-recycle, is an abbreviation for the subtraction. Dry gas refers to the methane, ethane, propane and ethylene and propylene produced in the cracking process, expressed in terms of weight percent of the initial charge. Light naphtha is the product that boils between about F. and about F. The heavy naphtha is the product that boils between about 170 F. and about 410 F. The cracking activity of a carrier is expressed in terms of the percent by volume of a standard hydrocarbon charge which is cracked under specific operating conditions in the Cat. A test. This test is described by Alexander and Shimp in National Petroleum News, 36, page R-537 (August 2, 1944). The unit for rating the cracking activity of the material is called the Activity Index (A1.) The term initially unaged catalyst refers to catalyst which has never been used in a conversion before, i.e., fresh catalyst, a catalyst which has not been used since it was last regenerated. T he invention may also be operated with catalyst that has not been used for a substantial period of time even though it is not fresh or newly regenerated. The term sulfur compounds is used herein to refer chemical combinations of sulfur with other materials which result in compounds soluble in hydrocarbons such as hydrogen sulfide, and any of the various compounds of hydrogen, carbon and sulfur.

The catalysts utilizable herein are those described in copending application Serial Number 825,016, filed on July 6, 1959, now U.S. Patent 2,945,806. Briefly, these catalysts comprise between about 0.05 percent, by weight, and about 10 percent, by weight of the final catalyst, preferably between about 0.1 percent and about percent, by weight, of at least one metal of the platinum and palladium series, i.e., those having atomic numbers of 44- 46, inclusive, 76-78, inclusive, supported upon synthetic composites of two or more refractory oxides. The carrier is a synthetic composite of two or more refractory oxides of the elements of Groups IIA, IIIB, IVA and B of the Periodic Arrangement of Elements [1. Chem. Ed., 16, 409 (1939)]. These synthetic composites of refractory oxides must have an Activity Index of at least about 25. They can also contain halogens and other materials which are known in the art as promoters for cracking catalysts, or small amounts of alkali metals that are added for the purpose of controlling the Activity Index of the carrier. Non-limiting examples of the composites contemplated herein include silica-alumina, silica-zirconia, silicia-alumina-zirconia, alumina-boria, silica-alumina-fiuorine, and the like. The preferred support is a synthetic composite of silica and alumina containing between about 1%, by weight, and about 90%, by weight, of alumina. These synthetic composites of two or more refractory oxides can be made by any of the usual methods known to those skilled in the art of catalyst manufacture. An example of a method of preparing it is set forth in copending application Serial Number 825,016, now US. Patent 2,945,806.

The following example illustrates a method of preparing platinum-containing catalysts utilizable in the process of this invention:

EXAMPLE 1 A synthetic silica-alumina carrier or support containing percent by weight alumina was prepared by mixing an aqueous solution of sodium silicate (containing 158 g. per liter of silica) with an equal amount of an aqueous acid solution of aluminum sulfate containing 39.4 g. Al (SO and 28.6 g. concentrate H 80 per liter. This mixture of solutions was dropped through a column of oil, wherein gelation of the hydrogel was effected in bead form. The head hydrogel was soaked in hot water (about 120 F.) for about 3 hours. The sodium in the hydrogel was then removed by exchanging the gel with an aqueous solution of aluminum sulfate [1.5% Al (SO by weight] containing a small amount (0.2 percent by weight) of ammonium sulfate. The thus-exchanged hydrogel bead was water-washed. Then, it was dried in superheated steam (about 280-340 F.) for about 3 hours and, finally, calcined at 1300 F. under a low partial pressure of steam for about 10 hours.

The silica-alumina beads were then crushed to pass through a 14-mesh screen and the material retained on a 25-mesh screen (US. Standard Screen Series) was used for catalyst preparation. Two portions of the crushed, calcine carrier were then barely covered with aqueous solutions of chloroplatinic acid, of concentrations sufficient to produce the desired amount of metal in each finished catalyst. The excess solution was removed by centrifuging. The thus-impregnated carrier was then heated in a covered beaker at 230 F. for '16 hours. The catalyst was heated to 450 F. in a nitrogen atmosphere and then treated with hydrogen for two hours at 450 F. Prior to use, it was activated in hydrogen for two hours at about 900 F.

One catalyst thus-prepared contained 0.47 percent platinum, by weight of the catalyst, and the silica-alumina carrier had an Activity Index of 46. It is designated Catalyst X. The other catalyst contained 1.7 percent platinum, by weight of the catalyst, on the 46 Al. carrier. It is designated Catalyst Y.

Any hydrocarbon fractionhaving an initial boiling point of at least about 400 F., and preferably a 50 percent point of at least about 500 F. and an end-boiling point of at least about 600 F., is suitable as a charge stock for the process of this invention. Such charge stocks include gas oils, residual stocks, cycle stocks, whole topped crudes, and heavy hydrocarbon fractions derived by the destructive hydrogenation of coal, tars, pitches, asphalts, etc., such as, for example, middle oil.

As is well known to those skilled in the art, the distillation of higher-boiling petroleum fractions (above about 750 F.) must be carried out under vacuum, in order to avoid thermal cracking. Throughout the specification and claims, however, the boiling temperatures are expressed in terms of the boiling point at atmospheric pressure. In other words, in all instances, the boiling points of frac tions distilled under vacuum have been corrected to the boiling points at atmospheric pressure.

In general the process of this invention may be operated under reaction conditions which will effect hydrocracking of the charge with a net consumption of hydrogen. In general, this will involve a reaction temperature above about 400 F., a reaction pressure above about 100 p.s.i.g., a liquid hourly space velocity within the range about 0.1 to 10 and a mole ratio of hydrogen to hydrocarbon charge within the range about 2 to about 80. It is a feature of the catalysts with which this invention will find use that they may be operated to obtain very high conversions at reaction conditions more modest than those generally suggested in the art. This makes it possible to operate this process at temperatures well below those at which excessive thermal cracking takes place. This preferred operation might be conducted at reaction temperatures within the range about 400 to 825 F. and hydrogen pressures within the range about 100 to 3000 p.s.ig. Still more preferably the reaction temperature will be within the range about 500 to 800 F., the hydrogen pressure within the range about 250 to 2000 p.s.i.g., the space velocity within the range about 0.1 to 4 and the mole ratios of hydrogen to hydrocarbon charge within the range about 5 to about 50.

As indicated above, one of the critical steps in applicants process is the addition of sulfur compounds to the charge during the initial on-stream period. The advantages of this may be demonstrated by the following example.

EXAMPLE 2 A gas oil having the following properties was hydrocracked.

Gravity, API 26.8 Distillation:

IBP 602 50% 781 885 Sulfur, wt. percent 0.79

This charge stock was hydrocracked, over a catalyst comprising 0.5 wt. percent platinum deposited of an active base having a 46 activity index, in several runs one without addition of sulfur compounds and the others with such addition.

Table 1 Run N Sulfur added, wt. percent of 0 {1 2 at 62 hr 3.8 at 56 hr charge. 1 8 at 98 hr 2.6 at 184 hr Total hrs. with added sulfur 0 130 184. Total hrs. to material balance 247 241 240. Material balance:

Average catalyst temp, F. 796 4 797. Yield 05+ product, vol. 106. 8 108.3 110.4

percent. Conversion to product boil- 58. 1 60.6 58.7

ing below 390 F., vol. percent. Conversion to product boil- 94. 2 97.8 98.0.

ing below 650 F., vol. percent. Estimated temp, F. for 797 791 794.

95 vol. percent conversion to products boiling below 650 F.

NOTE.I]1 run 3 the catalyst was preconditioned by subjecting it to a gas stream containing 0.5 mol percent hydrogen sulfide.

Several points may be made about this data. First of all, a greater liquid recovery is obtained when sulfur compounds are added during the start-up period than when they are not. Thus, in run 2. the yield of C product is 1.5% greater than in run 1 and in run 3 this difierence is increased to 3.6%. As an indication of the importance of this difierence it may be noted that one percent of added liquid yield over fuel gas in a 50,000 barrels per day refinery may amount to as much as $1500 per day or $550,000 per year. It will be noted in both runs 2 and 3 that at least a part of the increased liquid yield is reflected in increased gasoline production.

' Table I also shows reduced aging when sulfur compounds are added during start-up periods. This is demonstrated by the estimated temperature for 95% conversion in the last line of the table. This estimate is made by correcting the actual average reaction temperature by 1 F. for each 1% of the difference between the true 650 F. conversion and 95% Experience has shown this figure to be quite accurate.

. It is known that the greater the temperature needed to reach a given conversion level when using a specific catalyst the more the catalyst has aged. Table I demonstrates that the addition of sulfur compounds during the start-up period reduces the aging rate of the catalyst.

The amount of sulfur compounds which may be added to the charge stock to accomplish increased yields and reduced aging rates will generally be such as to raise the sulfur content of the hydrocarbon charge to a level within the range 0.5 to 5 wt. percent and preferably 1 to 5 wt. percent. The optimum amount to add in any given case will depend on a variety of factors such as the nature of the charge stock, its sulfur content and boiling range, the particular hydrocracking reaction conditions to be used, and the particular catalyst employed. The amount of sulfur compounds added can be varied with on-stream time. Generally, it may be advantageous to reduce the amount added gradually with time.

Y It has been found that the amount of sulfur compounds used should be correlated with the content of platinum series metal in the catalyst. The higher the content of platinum series metal, the higher the amount of sulfur compounds that should be added. This is demonstrated in the following examples. Two gas oil charge stocks were used in the runs described in the examples. The pertinent properties are set forth in Table II. It will be noted that the gas oils are almost identical in constitution, except for a difference in sulfur content.

EXAMPLE 3 Gas Oil A (0.13% S) was subjected to cracking in the presence of hydrogen and of Catalyst Y (1.7 percent Pt). The run was carried out continuously for fourteen hours, at a constant temperature of about 608 F. The hydrogen pressure was 1000 p.s.i.g. and the hydrogen to oil molar ratio was 40. The liquid hourly space velocity used was one. At intervals during the run, the amount of conversion into products boiling at temperatures lower than about 410 F. and the product distribution were determined. The pertinent data are set forth in Table III.

EXAMPLE 4 Gas Oil B (1.14 percent S) was subjected to a continuous 14-hour cracking operation in the presence of hydrogen and Catalyst Y (1.7 percent Pt). The temperature throughout the run was constant at about 615 F. The other reaction conditions were the same that were used in the run described in Example 3. Pertinent data at various stages of this run are set forth in Table III.

Table III Gas oil A (Example 3) Gas oil B (Example 4) Time, hrs 2 6 10 14 2 6 1O 14 Conversion, vol.

percent 53.2 49.9 42.7 29.0 62.4 61.6 57.4 52.2 Dry gas, wt.

percent 0.9 1.0 0.9 0.9 2.7 2.2 2.3 1.6 Butanes, Vol.

percent 4.1 7.1 5.4 3.8 6.3 12.7 12.3 11.4 Pentanes, v01.

percent 5.3 4.5 3.7 1.7 5.9 5.8 5.3 6.5 0 naphtha,

vol. percent 47.7 46.1 45.2 27.2 56.6 52.8 49.7 46.0 Cycle stlc, vol a percent 46.8 50.1 57.3 71.0 37.6 38.4 42.6 47.8 Wt. percent carbon on catalyst 7. 29 1. 60

1 Conversion into products boiling below 410 F.

The curves in FIGURE 1 are based upon the data set forth in Table III. Curve 1 defines the relationship between the volume percent conversion into products boiling at temperatures lower than about 410 F. and the time on-stream when the low sulfur containing gas oil (Gas Oil A) is cracked in the presence of the catalyst of relatively high platinum content. .Curve Zdefines the same relationship in the case of cracking a relatively high sulfur containing gas oil (Gas Oil B) in the presence of the same catalyst. It will be noted (Curve 1) that with a combination of high platinum content of the catalyst and low sulfur content of the charge stock, the conversion at constant temperature has declined from about 55 volume percent to about 29 volume percent in 14 hours, i.e., a drop of about 47 percent of .the original conversion. On the other hand, with a combination of relatively high platinum content in the catalyst with a relatively high sulfur content in the charge, the amount of conversion has dropped only from about 63 volume percent to about 53 volume percent in the same period of time. This amounts to a decline in conversion of only about 16 percent. It will be apparent, therefore, that the use of a high sulfur content in the charge has beneficiated the cracking activity in the presence of the catalyst of higher platinum content. When the platinum content of the catalyst is relatively low, the sulfur content of the charge can be lower. This is demonstrated by the following example.

EXAMPLE Gas Oil A (0.13 percent sulfur) was cracked in the presence of Catalyst X (0.47 percent Pt) at a constant temperature of about 605 F., in a run of 14 hours duration. This run also was carried out under the same conditions of space velocity, pressure, and hydrogen-oil ratio used in the run described in Example 3. The pertineut data are set forth in Table IV.

1 Conversion into products boiling below 410 I Curve 3 in FIG. 2 is based upon the data set forth in Table IV. This curve shows the relationship between the volume percent conversion into products boiling at temperatures lower than about 410 F. and the time onstream, when a gas oil of low sulfur content is cracked in the presence of a catalyst of relatively low platinum content. For comparison purposes, Curve 1 of FIG. 1 is included in FIG. 2. This curve shows the same relationship when the same gas oil is cracked in the presence of a catalyst of relatively high platinum content.

The optimum time of sulfur addition will vary with the particular amount of sulfur, the charge stock, catalyst and reaction conditions. Generally, this time may extend from the start-up to a time 0.5 hour to 14 days after start-up. More particular, the point where addition of sulfur compounds is stopped will usually be from 0.5 to days after start-up. Routine experimentation will establish the optimum time for any given system. Addition of sulfur compounds after that time will be disadvantageous.

In general, this invention will not be useful with charge stocks having a sulfur content more than 4% Any desired technique of adding sulfur compounds may be used. The amount of hydrogen sulfide in the gas stream supplied to the reactor may be varied to adjust, in

turn, the amount of hydrogen sulfide which dissolves in the charge stock. A high sulfur hydrocarbon fraction may be blended with the regular charge stock to produce a blend of optimum sulfur content. Sulfur compounds, such as thiophene, alkylthiophene, alkyl sulfides (e.g., butyl sulfide), thiophenols, mercaptans, and the like, may be added directly.

As is used herein and in the claims, the terms, percent sulfur and sulfur content of the charge, refers to the total sulfur content of the hydrocarbon material that is contacted with the catalyst, expressed as elemental sulfur, regardless of whether the sulfur content is the natural sulfur content of the charge or whether it is adjusted with sulfur compounds as described hereinbefore. It is to be noted that the process of this invention is dissimilar to prior art processes that involve the use of sulfur to maintain the valence state of a catalyst, such as in the case of a nickel-tungsten sulfide catalyst.

Although the present invention has been described with preferred embodiments, it is to be understood that modifications and variations may be resorted to, without departing from the spirit and scope of this invention, as those skilled in the art will readily understand. Such variations and modifications are considered to be within the purview and scope of the appended claims.

We claim:

1. A process for hydrocracking a hydrocarbon fraction having an initial boiling point of at least about 400 F. and a sulfur content of less than 4 percent by weight to produce lower boiling products which comprises contacting said hydrocarbon fraction in the presence of hydro gen for an extended period of time with an initially unaged catalyst comprising between about 0.05 percent and 10 percent, by weight of the catalyst, of at least one noble metal of the platinum and palladium series deposited upon a synthetic composite of refractory oxides of at least two elements of Groups IIA, IIIB and IV of the Periodic Arrangement of Elements having an activity index of at least 25 under hydrocracking reaction conditions to convert said hydrocarbon fraction to lower boiling products with a net consumption of hydrogen; adding substantial quantities of sulfur compounds to said hydrocarbon fraction to obtain a sulfur content in the range from about 0.5 to about 5 weight percent of said hydrocarbon fraction during a period of time which extends from the initial contacting of the hydrocarbon fraction with the catalyst to a time within the range from about 0.5 hour to about 14 days after said initial contacting; discontinuing said addition of sulfur compounds after said period of time has expired; and continuing the conversion of said hydrocarbon fraction without added sulfur compounds until such time as the catalyst becomes sufficiently aged as to require regeneration.

2. A process for hydrocracking a hydrocarbon fraction having an initial boiling point of at least about 400 F. and a sulfur content of less than 4 percent by weight to produce lower boiling products which comprises contacting said hydrocarbon fraction in the pres ence of hydrogen for an extended period of time with an initially unaged catalyst comprising between about 0.05 percent and 10 percent, by weight of the catalyst, of at least one noble metal of the platinum and palladium series deposited upon a synthetic composite of refractory oxides of at least two elements of Groups IIA, IIIB and IV of the Periodic Arrangement of Elements having an activity index of at least 25 under hydrocracking reaction conditions to convert said hydrocarbon fraction to lower boiling products with a net consumption of hydrogen; adding substantial quantities of hydrogen sulfide to said hydrocarbon fraction to obtain a sulfur content in the range from about 0.5 to about 5 weight percent of said hydrogen fraction during a period of time which extends from the initial contacting of the hydrocarbon fraction with the catalyst to a time within the range from about 0.5 hour to about 10 days after said initial contacting; discontinuing said addition of sulfur compounds after said period of time has expired; and continuing the conversion of said hydrocarbon fraction without added sulfur compounds until such time as the catalyst becomes sutficiently aged as to require regeneration.

3. A process for hydrocracking a hydrocarbon fraction having an initial boiling point of at least about 400 F. and a sulfur content of less than 4 percent by weight to produce lower boiling products which comprises contacting said hydrocarbon fraction in the presence of hydrogen for an extended period of time with an initially unaged catalyst comprising between about 0.05 percent and 10 percent, by weight of the catalyst, of platinum deposited upon a synthetic composite of refractory oxides of at least two elements of Groups IIA, H113 and IV of the Periodic Arrangement of Elements having an activity index of at least 25 under hydrocracking reaction conditions to convert said hydrocarbon fraction to lower boiling products with a net consumption of hydrogen; adding substantial quantities of sulfur comtime as the catalyst becomes sufficiently aged as to repounds to said hydrocarbon fraction to obtain a sulfur quire regeneration.

content in the range from about 0.5 to about 5 weight percent of said hydrocarbon fraction during a period References Cited in the file of this Patent of time which extends from the initial contacting of 5 UNITED STATES PATENTS the hydrocarbon fraction with the catalyst to a time within the range from about 0.5 hour to about 14 days {g p et z g8 after said initial contacting; discontinuing said addition 2'861944 5 2; g -g 33 1958 of sulfur compounds after said period of tune has ex- 2,863,825 Engel Dec. 9 1958 pired; and continuing the conversion of said hydrocar- 10 bon fraction without added sulfur compounds until such 2945806 Clapetta July 1960 

1. A PROCESS FOR HYDROCRACKING A HYDROCARBON FRACTION HAVING AN INITIAL BOILING POINT OF AT LEAST ABOUT 400*F. AND A SULFUR CONENT OF LESS THAN 4 PERCENT BY EIGHT TO PRODUCE LOWER BOILING PRODUCTS WHICH COMPRISES CONTACTING SAID HYDROCARBON FRACTION IN THE PRESENCE OF HYDROGEN FOR AN EXTENDED PERIOD OF TIME WITH AN INITIALLY UJNAGED CATALYST COMPRISING BETWEEN ABOUT 0.05 PERCENT AND 10 PERCENT, BY WEIGHT OF THE CATALYST, OF AT LEAST ONE NOBLE METAL OF THE PLATINUM AND PALLADIUM SERIES DEPOSITED UPON A SYNTHETIC COMPOSITE OF REFRACTORY OXIDES OF AT LEAST TWO ELEMENTS OF GROUPS IIA, IIIB AND IV OF THE PERIODIC ARRANGEMENT OF ELEMENTS HAVING AN ACTIVITY INDEX OF AT LEAST 25 UNDER HYDROCRACKING RREACTION CONDITIONS TO CONVERT SAID HYDROCARBON FRACTION TO LOWER BOILING PRODUCTS WITH A NET CONSUMPTION OF HYDROGEN; ADDING SUBSTANTIAL QUANTITIES OF SULFUR COMPOUNDS TO SAID HYDROCARBON FRACTION TO OBTAIN A SULFUR CONTENT IN THE RANGE FROM ABOUT 0.5 TO ABOUT 5 WEIGHT PERCENT OF SAID HYDROCARBON FRACTION DURING A PERIOD OF TIME WHICH EXTENDS FROM THE INITIAL CONTACTING OF THE HYDROCARBON FRACTION WITH THE CATALYST TO A TME WITHIN THE RANGE FROM ABOUT 0.5 HOUR TO ABOUT 14 DAYS AFTER SAID INITIAL CONTACTING; DISCONTINUING SAID ADDITION OF SULFUR COMPOUNDS AFTER SAID PERIOD OF TIME HAS EXPIRED; AND CONTINUING THE CONVERSION OF SAID HYDROCARBON FRACTION WITHOUT ADDED SULFUR COMPOUNDS UNTIL SUCH TIME AS THE CATALYST BECOMES SUFFICENTLY AGED S TO REQUIRE REGENERATION. 